A Home Subscriber Server (HSS) is a centralized subscriber database system which stores authentication information of a subscriber, service information of the subscriber, roaming information of the subscriber, relevant information indicating which core network serves the subscriber, information relevant to an authorization function of the subscriber, etc. The HSS is also adapted to record and provide original accounting data of the subscriber for a sorting system for output of a bill. Stated otherwise, the HSS stores information relevant to Authentication, Authorization and Accounting (AAA) of the subscriber.
A centralized storage structure is adopted for an existing IP Multimedia Subsystem (IMS) architecture, and an HSS typically stores information of millions of subscribers and has to support 1000 queries and 100 modifications per second. This is demanding for performance of the HSS and consequently renders the HSS expensive. Further, to guarantee 99.999% reliability of a system at a telecommunications level, the HSS utilizes at least a primary server and a backup server, and the backup server takes over the load originally imposed over the primary server when the primary server fails, which consequently doubles already extremely high hardware cost; and a delay may occur to activate the backup server after the primary server fails if the backup server is not located at the same physical address as the primary server, and the backup server may be prone to a failure when the primary server fails if the backup server is located at the same physical address as the primary server. Alike, as a result of such a centralized storage approach in the HSS, storage performance of the IMS architecture can typically be improved only by hardware improvements, which nevertheless is both difficult and inconvenient to a network topology, and consequently it is difficult to cater to increasing demands of subscribers.
Additionally, a roaming location and a home location are distinguished in a solution for the existing IMS architecture, and an HSS provides subscriber data for a local Serving Call Session Control Function (S-CSCF) only; and when roaming to another location, a subscriber has to firstly register with a Visited Location Register (VLR) at the roaming location and then obtain subscriber information from the HSS at the home location through the VLR, and this implementation process is burdensome and requires a complex signaling flow.